1. Field of the Invention
The present invention relates to a gain-variable amplifier used as an automatic gain control (AGC) circuit or the like.
2. Description of the Related Art
A first prior art gain-variable amplifier includes a first differential amplifier formed by first and second transistors having a common emitter connected a constant current source, a second differential amplifier formed by third and fourth transistors connected to the collector of the second transistor, and a third differential amplifier formed by fifth and sixth transistors having a common emitter connected to the collector of the first transistor. The first differential amplifier receives an input voltage. Also, the collectors of the third and fourth transistors are connected to the fifth and sixth transistors, respectively. The second and third differential amplifiers are controlled by a gain control voltage. This will be explained later in detail.
In the first prior art gain-variable amplifier, since the gain characteristics have an inflection point, it is difficult to control the gain by the gain control voltage. Also, the phase of the output voltage V.sub.out is inverted at the inflection point. Since only half of the exploitable range of the gain control voltage is utilized, the range of the gain control voltage is substantially reduced.
A second prior art gain-variable amplifier includes a first differential amplifier formed by first and second transistors having a common emitter connected to a first constant current source, a second differential amplifier formed by third and fourth transistors connected to the collector of the second transistor, and a third differential amplifier formed by fifth and sixth transistors having a common emitter connected to a second constant current source. Also, in this case, the first differential amplifier receives an input voltage. Also, the collectors of the third and fourth transistors are connected to the fifth and sixth transistors, respectively. The second and third differential amplifiers are controlled by a gain control voltage. This will be explained later in detail (see JP-A-3-32209).
In the second prior art gain-variable amplifier, since the gain characteristics have no inflection point, it is easy to control the gain by the gain control voltage. Also, the phase of the output voltage is never inverted. Since a full range of the exploitable range of the gain control voltage is utilized, the range of the gain control voltage is substantially increased.
In the second prior art gain-variable amplifier, the deviation of the DC component of the output voltage depending upon the gain control voltage is large. This large deviation can become enormous in the manufacture of an integrated circuit due to deviations in the current amplification factor.
Further, a signal may be subjected to distortion and degradation due to the difference in collector-emitter voltage between the first and second transistors. For example, in a radio transmitter/receiver sets, the distortion appears as harmonic distortion of a transmitting/receiving frequency which degrades the operating characteristics.